Honestly, I think it's because the entire series has retained it's top quality with every release, and the fact that every zelda ties in with the rest. They've never skipped out on gameplay, the story's are always very polished, and every one has reatained that element of immersion you don't see very often. A lot of it is smart business, too. I mean, the link you play has always been ambiguise,
exploration has been a priority, ill-recieved/ over-used characters are phased out, and combat ESPECIALLY with SS has seen a lot of innovation to keep things fresh. I don't believe there's a such thing as a BEST game, and Zelda does have its flaws. Its not always the best and has had a few "OK" Zelda games, but Zelda also has it's gems. I honestly believe that if they can keep raising the quality, the LOZ series will be the BEST series. But hey, a little competition never hurt.